METHOD AND DEVICE FOR PRECIPITATING CRYSTALS FROM A SOLUTION

Abstract: A method and a device for recovering crystals from a solution by evaporation with crystallisation in an evaporation plant having at least two stages are disclosed. In the method, the crystallised salt is separated in each evaporation stage (n) from mother liquor and is, in all stages except the first, dissolved to a substantially saturated solution, which is then conducted to the preceding evaporation stage (n-1). The crystallised salt in the first evaporation stage of the plant is collected as final product. The device comprises at least two evaporators (I1, I2, I3) with an inlet (1, 2, 3) for heating medium, and an inlet (6, 11, 14) for the solution to be evaporated, and an outlet (7, 12, 16) for the evaporated solution. The outlet of each evaporator (I1, I2, I3) is provided with separating means (C1, C2, C3) for separating crystals from mother liquor. The inlet of an evaporator (I1, I2...In-1) is connected to the separating means (C2, C3...Cn) of the subsequent evaporator (I2, I3...In) for receiving crystals from the separating means, and to a supply means (5) for liquid which forms, with the crystals, a substantially saturated solution which is to be evaporated. The outlet (10) for crystals from the separating means (C1) of the first evaporator (I1) of the device is the product outlet.

Description: METHOD AND DEVICE FOR PRECIPITATING CRYSTALS FROM A SOLUTION The present invention relates to a method and a device for recovering crystals from a solution by evapo¬ ration with crystallisation in an evaporation plant hav¬ ing at least two stages. The technique can be generally used in applications where a product in the form of crystals is to be recover¬ ed from a solution. A particularly preferred field is the recovery of NaCl in connection with flue gas cleaning in which hydrogen chloride is absorbed in an aqueous solu- tion and, after treatment with a sodium-based salt, e.g. sodium hydroxide, sodium bicarbonate and sodium carbo¬ nate, having a pH-increasing effect, is present as dis¬ solved NaCl together with a number of impurities. The present invention will now be described with reference to this specific and especially preferred application, although the invention is not restricted to this field, but concerns all applications in which crystals are recovered from a solution. Since the requirements as regards pollution-control measures are becoming more and more stringent, it is desirable to carry out flue gas cleaning in an essential¬ ly closed system, i.e. no polluted waste water should be discharged. One step in providing such a closed system is to separate, in wet collecting of hydrogen chloride from flue gases, hydrogen chloride as a dilute solution which after treatment with a sodium-based salt contains, in addition to NaCl, several different impurities. By evapo¬ ration and crystallisation, the NaCl-content of the solu¬ tion is recovered in as pure a form as possible. The eva- porated water is condensed and recycled. Evaporation and crystallisation of the solution can be carried out in various ways. A frequent technique is so-called single-effect evaporation, i.e. evaporation and crystallisation are carried out in a single evaporator. This technique requires, however, a considerable consump¬ tion of steam and results in inferior salt purity. The inferior salt purity depends on, inter alia, the high concentration of calcium impurities in the incoming liquid and the insufficient capacity of the technique to separate NaCl and impurities. One way of reducing the steam consumption is to per¬ form the evaporation in several steps, one evaporator being used for each partial step in the production of salt. A prior-art embodiment comprises a plant with four evaporators, preevaporation being performed in a first evaporator, crystallisation of product salt or salt for recrystallisation in a second evaporator and crystalli- sation of contaminated salt in a third evaporator. The final concentration of waste is carried out in a fourth evaporator without recovery of salt. If recrystallisation of product salt is necessary, the plant is equipped with a further evaporator. In order to avoid problems with coatings in the wet collecting of hydrogen chloride from flue gases, the con¬ centration of dissolved substances in the absorption solution is low after treatment with a sodium-based salt, i.e. it consists of a dilute NaCl-solution. This means that a large amount of water must be distilled off by preevaporation before the crystallisation of NaCl begins. When the water in the solution is distilled off, it is however not only the NaCl concentration that increases, but also the concentration of the impurities, which means that the crystallised salt is relatively contaminated. Therefore, a recrystallisation step is in many cases required so as to obtain a higher purity of the product salt. The salt which is to be recrystallised is usually dissolved in condensate and supplied to an additional evaporator, which entails an additional steaming cost in the plan . The object of the present invention is to provide a method and a device for obtaining, with a low steam con¬ sumption, a salt of very high purity, without necessitat¬ ing a separate recrystallisation step. This object is achieved by carrying out the evapora¬ tion in a plant with at least two stages, the salt which is crystallised in each stage except in the first one being redissolved to a substantially saturated solution, i.e. a solution having a degree of saturation of 20-100%, preferably 80-100%, especially 98%, and subsequently being supplied as a solution for evaporation to the pre¬ ceding stage in the plant. As a result, a successive recrystallisation in several steps occurs. The final pro¬ duct salt is collected from the first evaporator of the plant, in which the lowest concentration of impurities is to be found as the salt is crystallised. According to the invention, a method thus is provid¬ ed for recovering crystals from a solution by evaporation with crystallisation in an evaporation plant having at least two stages, the crystallised salt in each evapora¬ tion stage (n) being separated from mother liquor, char¬ acterised in that the crystallised salt, in every stage except in the first one, is dissolved to a solution with a degree of saturation of 20-100%, whereupon this is con- ducted to the preceding evaporation stage (n-1), and that the crystallised salt in the first evaporation stage of the plant is discharged as final product. According to the invention, there is also provided a device for recovering crystals from a solution by evapo- ration with crystallisation in an evaporation plant, said device comprising at least two evaporators with an inlet and an outlet for a heating medium and an inlet for the solution to be evaporated and an outlet for the evapo¬ rated solution, the outlet of each evaporator being pro- vided with separating means for separation of crystals from mother liquor, characterised in that the inlet of an evaporator (n-1) is connected to the separating means of the subsequent evaporator (n) for receiving crystals from the separating means, and to a supply means for a liquid which forms, with the crystals, a solution having a degree of saturation of 20-100% which is to be evapo- rated, and that the outlet for crystals from the sepa¬ rating means of the first evaporator of the device is the product outlet. Further features and advantages of the invention are defined in the following specification and in the claims. The invention will now be described in detail with reference to the accompanying drawing which schematically illustrates a preferred evaporation plant according to the invention. In the below description, the expression "preceding evaporator" concerns the evaporator which, with respect to the liquid flow in the plant, is arranged immediately upstream of another evaporator. The expres¬ sion "subsequent evaporator" concerns the evaporator which, with respect to the liquid flow in the plant, is arranged immediately downstream of another evaporator. The expression "first evaporator" concerns the evaporator which, with respect to the liquid flow in the plant, is the first in the plant. No separated mother liquor is fed to this evaporator. Moreover, the term "mother liquor" refers to the liquid which is separated from crystals formed in evaporation, and finally the term "degree of saturation" refers to the relation of the concentration of the solution to the concentration of the saturated solution. The plant in the Figure comprises three evaporators II, 12 and 13, three separating means Cl, C2 and C3, two containers Tl and T2 and one condenser Kl. The three evaporators II, 12 and 13 are connected to the conduits 1, 2 and 3 for supplying the heating medium which preferably is steam, and the evaporator 13 is con- nected to the condenser Kl via the conduit 4. The conduit 5, which feeds, from a store (not shown), an incoming NaCl-containing solution to the plant, is connected to the containers Tl and T2. The container Tl is further connected to the evaporator II via the conduit 6 for conveying solution for evaporation, and the evaporator II has an outlet 7 for evaporated solution, which is connected to the separating means Cl via the conduit 8. The separating means Cl comprises an outlet 9 for mother liquor, which is connected to the evaporator 12, and an outlet 10 for separated product crystals. The container T2 is connected to the evaporator 12 via the conduit 11 for conveying solution to be evapo¬ rated. The evaporator 12 has an outlet 12 which is con¬ nected to the separating means C2 via the conduit 13. The separating means C2 has an outlet 14 for mother liquor, which is connected to the evaporator 13, and an outlet 15 which is connected to the container Tl for conveying separated crystals. The evaporator 13 comprises an outlet 16 connected to the separating means C3 via the conduit 17. The sepa- rating means C3 has an outlet 18 for mother liquor which is discharged as waste. Furthermore, the separating means C3 comprises an outlet 19 which is connected to the con¬ tainer T2 for conveying separated crystals. The plant described above functions as follows. Of the incoming solution LI to the plant, a partial amount Lla is used to be dissolved to a substantially saturated solution of crystallised salt S2 from the eva¬ porator 12. The substantially saturated solution is fed into the first evaporator II of the plant, in which a product salt SI is crystallised. This product salt SI is separated from mother liquor L2 in the separating means Cl, whereupon the mother liquor L2 is supplied to the evaporator 12. To the evaporator 12 there is fed a substantially saturated solution, which is formed of crystallised salt S3 from the evaporator 13 and a partial amount Lib of the incoming solution LI to the plant. The salt S2 crystal- used in the evaporator 12 is separated from mother liquor L3 in the separating means C2, whereupon the salt S2 is conducted to the container Tl to be dissolved, and the mother liquor L3 is conducted to the evaporator 13. The salt S3 crystallised in the evaporator 13 is separated from mother liquor L4 in the separating means C3, whereupon the salt S3 is conducted to the container T2 to be dissolved, and the mother liquor L4 is discharg¬ ed as waste. In order to reduce the amount of waste, the mother liquor L4 can be e.g. spray-dried before being stored. In order to perform the evaporation in the plant, a heating medium HI (steam) is fed to the evaporator II via the conduit 1. The heating medium, which is produced in the evaporator II, is transferred to the evaporator 12 in the conduit 2, and the one produced in the evaporator 12 is fed to the evaporator 13 in the conduit 3. Finally, the heating medium which is produced in the evaporator 13 is transferred to the condenser Kl to be condensed. The condensate formed can be transferred to a boiler for producing fresh heating medium in the form of steam and/or to a preceding wet collector. The method described above and schematically illu¬ strated in Fig. 1 can be varied in several ways within the scope of the appended claims. Thus, instead of the solution LI fed to the plant, pure water from a store (not shown) can be added when dissolving the salt S2 obtained from the evaporator 12. This is done if a par¬ ticularly pure product salt SI is desirable. To separate impurities from the solution fed to the evaporator II, an ion exchanger or a means for chemical precipitation can be arranged immediately before the sup¬ ply of the solution to the evaporator II. Furthermore, two evaporators can be connected in parallel for the heating medium (steam), whereas the conveying of mother liquor and salt is carried out as described above. Moreover, the plant can have concurrent or counter- current feeding, concurrent feeding implying that the solution and the heating medium accompany each other from evaporator to evaporator, as illustrated in Fig. 1. Coun- tercurrent feeding means that the solution flows e.g. from the left to the right in Fig. 1, whereas the heating medium flows from the right to the left. This means that the incoming heating medium to the plant is introduced into the evaporator 13, and that the heating medium pro- duced in the evaporator 13 is transferred to the evapo¬ rator 12 etc. The heating medium produced in the evapora¬ tor II is transferred to a condenser. The mother liquor separated in the separating means Cl and C2 can be conducted not only to the directly sub- sequent evaporator 12 and 13, respectively, but also from e.g. Cl to 13 (not shown in the drawing). However, the separated mother liquor should not be conducted upstream with respect to the liquid flow in the plant, i.e. the mother liquor separated in e.g. the separating means C2 should not be recycled to the evaporator 12 or II, since this would increase, in an undesirable manner, the con¬ tent of impurities of the liquid supplied to these eva¬ porators. The number of evaporators which the inventive device can have, is determined by the steam data of the heating medium which is fed to the first evaporator, as well as the conditions (temperature, pressure) under which the condenser at the end of the device operates. While apply¬ ing the existing technique, it is possible from an econo- mic view to have a maximum of 6 evaparators, but prefer¬ ably 3-4 evaporators are used. The separating means in the device may consist of e.g. centrifuges, screens or sieves. At present, centri¬ fuges are preferred. For the purpose of exemplifying the present inven¬ tion, a test was carried out in a plant according to the Figure. The incoming heating medium used in the plant was saturated steam at a pressure of 2 bar, and the incoming solution to the plant was composed as stated in Table 1. The results from the test are shown in Table 1. In order to compare these results with those obtain- ed from a plant according to the prior-art technique, a test was carried out in which the incoming solution to the plant was the same as in the above test. Also in this case, the incoming heating medium was saturated steam at a pressure of 2 bar. The prior-art plant comprises three evaporators of same type as in the inventive plant described above, pre- concentration being carried out in the first evaporator. A partial flow of the evaporated solution from the third evaporator was mixed with the preconcentrated solution from the first evaporator and conducted to the second evaporator from which the product salt was collected. In the test according to the prior-art technique, the results shown in Table 1 were obtained. TABLE 1 (% by weight) NaCl KCL K2SO4 NaBr NaF CaSθ4 H2O Incoming solution to the plant 8.33 0.17 0.26 0.062 0.0042 0.0021 91.17 Product salt in prior- art technique 93.37 0.57 0.90 0.21 0.014 0.0072 4.95 Product salt in connection with the present invention 94.46 0.18 0.28 0.068 0.0045 0.0023 5.01 VD Waste in prior-art tech¬ nique 24.38 2. 68 4. 19 1 .00 0.067 0.034 67.73 Waste in connection with the present invention 22. 82 4. 63 7.24 1.74 0. 12 0.058 63.39 As is evident from Table 1, a much purer salt is obtained in the plant according to the present invention as compared to the prior-art plant. Thus, the present invention provides a method and a device for recovering crystals from a solution by eva¬ poration, a purer product salt being obtained at a lower cost, since a separate recrystallisation step is not necessary.

Claims: CLAIMS 1. Method for recovering crystals from a solution by evaporation with crystallisation in an evaporation plant having at least two stages, the crystallised salt in each evaporation stage (n) being separated from mother liquor, c h a r a c t e r i s e d in that the crystallised salt, in all stages except the first one, is dissolved to a solution having a degree of saturation of 20-100%, where¬ upon this is fed to the preceding evaporation stage (n-1), and that the crystallised salt in the first evapo¬ ration stage of the plant is collected as final product. 2. Method as claimed in claim 1, c h a r a c t e r - i s e d in that the crystallised salt is dissolved in the incoming solution to the plant to a solution having a degree of saturation of 80-100%, preferably 98%. 3. Method as claimed in claim 1 or 2, c h a r a c ¬ t e r i s e d in that the crystallised salt in the second evaporation stage of the plant is dissolved in pure water to a solution having a degree of saturation of 80-100%, preferably 98%. 4. Method as claimed in claim 3, c h a r a c t e r ¬ i s e d in that the salt solution, before being fed to the first evaporation stage, is conveyed through an ion exchanger and/or is subjected to chemical precipitation. 5. Method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that steam is supplied to the evaporation plant countercurrently or concurrently with the liquid. 6. Method as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that all mother liquors in each evaporation stage (n) are separated from crystallis¬ ed salt and, in all stages except the last one, are transferred to the evaporation stage (n+1) which with respect to the liquid flow is arranged downstream in the plant. 7. A device for recovering crystals from a solution by evaporation with crystallisation in an evaporation plant, said device comprising at least two evaporators (II, 12, 13) with an inlet (1, 2, 3) for a heating medium and an inlet (6, 11, 14) for a solution to be evaporated and an outlet (7, 12, 16) for the evaporated solution, the outlet (7, 12, 16) of each evaporator (II, 12, 13) being provided with separating means (Cl, C2, C3) for separating crystals from mother liquor, c h a r a c - t e r i s e d in that the inlet of an evaporator (II, I2...1n-1) is connected to the separating means (C2, C3...Cn) of the subsequent evaporator (12, 13...In) for receiving crystals from the separating means, and to a supply means (5) for liquid which forms, with the crys- tals, a solution which has a degree of saturation of 20-100% and is to be evaporated, and that the outlet (10) for crystals from the separating means (Cl) of the first evaporator (II) of the device is the product outlet. 8. Device as claimed in claim 7, c h a r a c t e r - i s e d in that the supply means ( 5) for liquid which forms, with the crystals, a solution which has a degree of saturation of 20-100% and is to be evaporated, is con¬ nected to a store containing the solution to be evapo¬ rated (LI ) . 9. Device as claimed in claim 7 or 8, c h a r a c ¬ t e r i s e d in that the supply means for the liquid adjacent to the inlet of the first evaporator of the device is connected to a water store. 10. Device as claimed in claim 9, c h a r a c - t e r i s e d in that the inlet ( 6) of the first eva¬ porator ( II ) of the device is equipped with an ion exchanger and/or means for chemical precipitation. 11. Device as claimed in any one of claims 7-10, c h a r a c t e r i s e d in that the heating medium inlet of an evaporator (In) is connected to the heat¬ ing medium outlet or inlet of the preceding evaporator (In-1).

Inventors: RAGNARSSON SVEN, RAGNARSSON, SVEN
Applicants: FLAEKT AB, RAGNARSSON SVEN, ABB FLAEKT AB, RAGNARSSON, SVEN
Application year: 1994
Main classification: B01D9
IPC codes: B01D1, B01D9